Improving air sparging systems by pulsed operation

Air sparging was evaluated for remediation of tetrachloroethylene (PCE) present as dense non-aqueous phase liquid (DNAPL) in aquifers. A 2-dimensional laboratory tank with a 50-cm high DNAPL zone was created, with a PCE pool accumulating on an aquitard. During air injection, the vadose zone DNAPL was removed within a few days, and the recovery in the extracted soil vapors decreased to low values. Pulsing the air injection led to improved mass recovery, as the pulsing induced water circulation and increased the DNAPL dissolution rate. The induced circulation of water led to limited spreading of the dissolved contaminant, but accelerated mass removal by 40 to 600%, depending on the aggressiveness of the pulsing. For field applications, pulsing with a daily or diurnal cycling time may increase mass removal, thus reducing the treatment time and saving in the order of 40 to 80% of the energy cost used to run the blowers. PCE DNAPL located below the sparge point was unaffected by the air sparging and cannot be removed using this technology.